Method for transmitting demodulation reference signals in wireless communication system and terminal using same

ABSTRACT

A method of transmitting an uplink reference signal of a user equipment (UE) in a multi-node system is described. The method according to an embodiment includes receiving a synchronization signal from a node; receiving a parameter for a virtual cell identifier (ID) from the node; generating an uplink demodulation reference signal (DM-RS) using the parameter for the virtual cell ID; and transmitting the generated uplink DM-RS to the node. A physical cell ID is a cell ID obtained from the synchronization signal, and the parameter for the virtual cell ID is a parameter used for generating the uplink DM-RS in the replacement of the physical cell ID.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. application Ser.No. 14/234,963 filed on Jan. 24, 2014, which is the National Phase ofPCT/KR2012/006023 filed on Jul. 27, 2012, which claims priority under 35U.S.C. 119(e) to U.S. Provisional Application No. 61/512,374 filed onJul. 27, 2011. The entire contents of all these applications are herebyexpressly incorporated by reference into the present application.

BACKGROUND OF THE INVENTION

The present invention relates to wireless communications, and moreparticularly, to an uplink reference signal transmission method formitigating an interference in a multi-node system, and a user equipmentusing the method.

A data transfer amount of a wireless network has been rapidly increasedin recent years. It is because various devices, e.g., a smart phone, atablet personal computer (PC), or the like, that requiremachine-to-machine (M2M) communication and a high data transfer amounthave been introduced and distributed. To satisfy the required high datatransfer amount, a carrier aggregation technique, a cognitive radiotechnique, or the like for effectively using more frequency bands and amultiple antenna technique, a multiple base station cooperationtechnique, or the like for increasing data capacity within a limitedfrequency have recently drawn attention.

In addition, the wireless network has been evolved in a direction ofincreasing density of nodes capable of accessing to an area around auser. Herein, the node implies an antenna (or antenna group) which isseparated from a distributed antenna system (DAS) by more than a certaindistance. However, the node is not limited to this definition, and thuscan also be used in a broader sense. That is, the node may be apico-cell evolved node B (PeNB), a home eNB (HeNB), a remote radio head(RRH), a remote radio unit (RRU), a relay, etc. A wireless communicationsystem having nodes with higher density can provide higher systemperformance through cooperation between the nodes. That is, bettersystem performance can be achieved when one base station controllermanages transmission and reception of respective nodes and thus thenodes operate as if they are antennas or an antenna group for one cell,in comparison with a case where the respective nodes operate as anindependent base station (BS), advanced BS (ABS), Node-B (NB), eNode-B(eNB), access point (AP), etc., and thus do not cooperate with eachother. Hereinafter, a wireless communication system including aplurality of nodes is referred to as a multi-node system.

In the multi-node system, a plurality of nodes can use one physical cellidentifier (ID). Accordingly, there is an advantage in that the numberof handover attempts is decreased, and cooperative communication betweenthe nodes becomes easy.

In the conventional technique, a user equipment (UE) generates variousuplink signals on the basis of a physical cell ID used by a BS or anode. However, an interference between uplink signals is increased inproportion to the number of UEs in a cell. In particular, aninterference between uplink reference signals may be problematic.

SUMMARY OF THE INVENTION

The present invention provides a method for transmitting an uplinkreference signal in a multi-node system, and a user equipment using themethod.

According to an aspect of the present invention, a method oftransmitting an uplink reference signal of a user equipment (UE) in amulti-node system is provided. The method includes: receiving asynchronization signal from a node; receiving a parameter for a virtualcell identifier (ID) from the node; generating an uplink demodulationreference signal (DM-RS) using the parameter for the virtual cell ID;and transmitting the generated uplink DM-RS to the node, wherein aphysical cell ID is a cell ID obtained from the synchronization signal,and the parameter for the virtual cell ID is a parameter used forgenerating the uplink DM-RS in the replacement of the physical cell ID.

In the aforementioned aspect of the present invention, the parameter forthe virtual cell ID may be a UE-specific parameter given differently foreach UE.

In addition, the uplink DM-RS may be generated by cyclically shifting abase sequence selected from one of a plurality of sequence groups, andeach of the plurality of sequence groups may include at least one basesequence.

In addition, the cyclic shift may be determined based on the parameterfor the virtual cell ID.

In addition, the uplink DM-RS may be transmitted in at least two slotsin a frame including a plurality of slots in a time domain, one sequencegroup may be selected in each slot of the at least two slots, and theuplink CM-FS may be generated by cyclically shifting one base sequenceselected from the selected one sequence group.

In addition, the one sequence group selected for each slot may bedetermined based on the parameter for the virtual cell ID.

In addition, the base sequence selected from the one sequence groupdetermined for each slot may be determined based on the parameter forthe virtual cell ID.

In addition, the parameter for the virtual cell ID may include a virtualcell ID having any one of integer values ranges from 0 to 513, and thevirtual cell ID may be used to generate the uplink DM-RS in thereplacement of the physical cell ID.

In addition, the physical cell ID may be used to generate the remaininguplink signals other than the DM-RS.

In addition, the parameter for the virtual cell ID may be transmitted byusing a radio resource control (RRC) message.

In addition, the method may further include receiving uplink schedulinginformation from the node, wherein the parameter for the virtual cell IDis generated based on a parameter included in the uplink schedulinginformation.

In addition, the uplink scheduling information may include informationindicating a frequency band at which the UE transmits an uplink datachannel, and the frequency band may include a band overlapping with afrequency band of another UE for transmitting an uplink data channel andDM-RS simultaneously with the UE.

In addition, the DM-RS may be transmitted in 4^(th) and 11^(th) singlecarrier-frequency division multiple access (SC-FDMA) symbols in anuplink subframe including 14 SC-FDMA symbols.

In addition, the DM-RS may be transmitted in 3rd and 9^(th) SC-FDMAsymbols in an uplink subframe including 12 SC-FDMA symbols.

According to another aspect of the present invention, a UE fortransmitting an uplink DM-RS in a multi-node system is provided. The UEincludes: a radio frequency (RF) unit for transmitting and receiving aradio signal; and a processor coupled to the RF unit, wherein theprocessor is configured for: receiving a synchronization signal from anode; receiving a parameter for a virtual cell ID from the node;generating an uplink DM-RS using the parameter for the virtual cell ID;and transmitting the generated uplink DM-RS to the node, wherein aphysical cell ID is a cell ID obtained from the synchronization signal,and the parameter for the virtual cell ID is a parameter used forgenerating the uplink DM-RS in the replacement of the physical cell ID.

A user equipment (UE) can generate an uplink signal by using a physicalcell identifier (ID) and a virtual cell ID additionally provided foreach UE. In particular, an uplink reference signal can be generatedbased on the virtual cell ID. According to the present invention, aninterference can be mitigated in comparison with a case where aplurality of UEs use the same physical cell ID to generate the uplinkreference signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a multi-node system.

FIG. 2 shows a multi-node system using the same physical cell identifier(ID).

FIG. 3 shows a structure of a radio frame in 3rd generation partnershipproject (3GPP) long term evolution (LTE).

FIG. 4 shows an example of a resource grid for one slot.

FIG. 5 shows a structure of a downlink subframe.

FIG. 6 shows an orthogonal frequency-division multiplexing (OFDM) symbolfor transmitting a synchronization signal and a physical broadcastchannel (PBCH) within a radio frame in a frequency division duplex (FDD)system.

FIG. 7 shows a structure of an uplink subframe.

FIG. 8 shows an exemplary structure of a subframe in which a referencesignal is transmitted.

FIG. 9 shows a demodulation reference signal (DM-RS) transmission methodaccording to an embodiment of the present invention.

FIG. 10 shows a structure of a base station and a user equipmentaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The technology described below can be used in various multiple accessschemes such as code division multiple access (CDMA), frequency divisionmultiple access (FDMA), time division multiple access (TDMA), orthogonalfrequency division multiple access (OFDMA), single carrier frequencydivision multiple access (SC-FDMA), etc. The CDMA can be implementedwith a radio technology such as universal terrestrial radio access(UTRA) or CDMA2000. The TDMA can be implemented with a radio technologysuch as global system for mobile communications (GSM)/general packetratio service (GPRS)/enhanced data rates for GSM evolution (EDGE). TheOFDMA can be implemented with a radio technology such as institute ofelectrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc. The UTRA is a part ofa universal mobile telecommunications system (UMTS). 3^(rd) generationpartnership project (3GPP) long term evolution (LTE) is a part of anevolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the OFDMA in adownlink and uses the SC-FDMA in an uplink. LTE-advance (LTE-A) is anevolution of the LTE.

FIG. 1 shows an example of a multi-node system.

Referring to FIG. 1, the multi-node system includes a base station (BS)and a plurality of nodes.

The BS is generally a fixed station that communicates with a userequipment (UE) and may be referred to as another terminology, such as anevolved Node-B (eNB), a base transceiver system (BTS), an access point,etc. The BS coupled to the plurality of nodes can control each node.

The node may imply a macro eNB, a pico-cell eNB (PeNB), a home eNB(HeNB), a remote radio head (RRH), a relay, a distributed antenna, etc.Such a node is also referred to as a point.

In the multi-node system, if one BS controller manages transmission orreception of all nodes and thus individual nodes operate as if they area part of one cell, then the system can be regarded as a distributedantenna system (DAS) which constitutes one cell. In the DAS, separatenode identifiers (IDs) may be given to the individual nodes, or theindividual nodes may operate as if they are some antenna groups within acell without the additional node IDs. In other words, the DAS is asystem in which antennas (i.e., nodes) are deployed in various positionswithin a cell in a distributed manner, and these antennas are managed bythe BS. The DAS is different from a conventional centralized antennasystem (CAS) in which antennas of the BS are concentrated in a cellcenter.

If the individual nodes have separate cell IDs and perform schedulingand handover in the multi-node system, the system can be regarded as amulti-cell (e.g., macro-cell/femto-cell/pico-cell) system. If themultiple cells are configured such that they overlap with each otheraccording to coverage, this is called a multi-tier network.

FIG. 2 shows a multi-node system using the same physical cell ID.

Referring to FIG. 2, a node 1 may be a macro eNB, and nodes 2 to 5 maybe RRHs. The nodes 1 to 5 may use the same physical ID.

A UE may transmit an uplink signal to a different node according to alocation thereof. For example, a UE 1 may transmit an uplink signal tothe node 2, and a UE 2 may transmit an uplink signal to the node 3. Assuch, when different UEs transmit uplink signals by using the same radioresource, it may cause a mutual interference. Each UE applies uplinkprecoding to mitigate the mutual interference, and each node uses areception signal processing method to mitigate the interference. Thismethod is also called multi-user multi input multi output (MU-MIMO).

When applying the MU-MIMO method, a BS or a node uses an uplinkdemodulation reference signal (DM-RS) to recognize a specific precodingmatrix used by the UE and a specific uplink channel experienced by theUE. The DM-RS is a reference signal related to an uplink data channel orcontrol channel transmitted by the UE.

Therefore, when a mutual interference does not exist as much as possiblebetween uplink DM-RSs transmitted by respective UEs, the BS or the nodecan correctly estimate an effective channel for each UE to remove theinterference, thereby facilitating data reception.

FIG. 3 shows a structure of a radio frame in 3GPP LTE.

Referring to FIG. 3, a radio frame includes 10 subframes. One subframeis defined as two consecutive slots. A time required for transmittingone subframe is called a transmission time interval (TTI). A time lengthof the radio frame is T_(f)=307200*T_(s)=10 ms, and consists of 20slots. A time length of the slot is T_(slot)=15360*T_(s)=0.5 ms, and isnumbered from 0 to 19. In frequency division duplex (FDD), a downlink inwhich each node or BS transmits a signal to a UE and an uplink in whichthe UE transmits a signal to each node or BS are divided in a frequencydomain. In time division duplex (TDD), a downlink and an uplink can usethe same frequency band between each node (or BS) and the UE, and can bedivided in a time domain.

FIG. 4 shows an example of a resource grid for one slot.

Referring to FIG. 4, one slot includes a plurality of orthogonalfrequency-division multiplexing (OFDM) symbols in a time domain, andincludes N_(RB) resource blocks in a frequency domain. Herein, one slotincludes 7 OFDMA symbols, and one resource block (RB) includes 12subcarriers in the frequency domain. However, this is for exemplarypurposes only, and thus the present invention is not limited thereto.

Each element on the resource grid is referred to as a resource element(RE). The RE on the resource grid can be identified by an index pair(k,l) within the slot. Herein, k(k=0, . . . , N_(RB)×12-1) denotes asubcarrier index in the frequency domain, and l(l=0, . . . , 6) denotesan OFDM symbol index in the time domain.

The number N^(DL) of RBs included in a downlink slot depends on adownlink transmission bandwidth determined in a cell.

FIG. 5 shows a structure of a downlink subframe.

Referring to FIG. 5, the downlink subframe is divided into a controlregion and a data region in a time domain. The control region includesup to first four OFDM symbols of a 1^(st) slot in the subframe. However,the number of OFDM symbols included in the control region may vary. Aphysical downlink control channel (PDCCH) and other control channels areallocated to the control region, and a physical downlink shared channel(PDSCH) is allocated to the data region.

As disclosed in 3GPP TS 36.211 V10.2.0, 3GPP LTE/LTE-A classifies aphysical channel into a data channel and a control channel. Examples ofthe data channel include a physical downlink shared channel (PDSCH) anda physical uplink shared channel (PUSCH). Examples of the controlchannel include a physical downlink control channel (PDCCH), a physicalcontrol format indicator channel (PCFICH), a physical hybrid-ARQindicator channel (PHICH), and a physical uplink control channel(PUCCH).

The PCFICH transmitted in a 1^(st) OFDM symbol of the downlink subframecarries a control format indicator (CFI) regarding the number of OFDMsymbols (i.e., a size of the control region) used for transmission ofcontrol channels in the subframe. The UE first receives the CFI on thePCFICH, and thereafter monitors the PDCCH.

Unlike the PDCCH, the PCFICH does not use blind decoding, and istransmitted by using a fixed PCFICH resource of the subframe.

The PHICH carries a positive-acknowledgement(ACK)/negative-acknowledgement (NACK) signal for an uplink hybridautomatic repeat request (HARQ). The ACK/NACK signal for uplink (UL)data on a PUSCH transmitted by the UE is transmitted on the PHICH.

Control information transmitted through the PDCCH is referred to asdownlink control information (DCI). The DCI may include resourceallocation of the PDSCH (this is referred to as a downlink (DL) grant),resource allocation of a PUSCH (this is referred to as an uplink (UL)grant), a set of transmit power control commands for individual UEs inany UE group, and/or activation of a voice over Internet protocol(VoIP).

The 3GPP LTE uses blind decoding for PDCCH detection. The blind decodingis a scheme in which a desired identifier is de-masked from a cyclicredundancy check (CRC) of a received PDCCH (referred to as a candidatePDCCH) to determine whether the PDCCH is its own control channel byperforming CRC error checking.

The BS determines a PDCCH format according to DCI to be transmitted tothe UE, attaches a CRC to the DCI, and masks a unique identifier(referred to as a radio network temporary identifier (RNTI)) to the CRCaccording to an owner or usage of the PDCCH.

A control region in a subframe includes a plurality of control channelelements (CCEs). The CCE is a logical allocation unit used to providethe PDCCH with a coding rate depending on a radio channel state, andcorresponds to a plurality of resource element groups (REGs). The REGincludes a plurality of resource elements. According to an associationrelation of the number of CCEs and the coding rate provided by the CCEs,a PDCCH format and the number of bits of a possible PDCCH aredetermined.

One REG includes 4 REs. one CCE includes 9 REGs. The number of CCEs usedto configure one PDCCH may be selected from a set {1, 2, 4, 8}. Eachelement of the set {1, 2, 4, 8} is referred to as a CCE aggregationlevel.

The BS determines the number of CCEs used in transmission of the PDCCHaccording to a channel state. For example, a UE having a good downlinkchannel state can use one CCE in PDCCH transmission. A UE having a poordownlink channel state can use 8 CCEs in PDCCH transmission.

A control channel consisting of one or more CCEs performs interleavingin an REG unit, and is mapped to a physical resource after performingcyclic shift based on a cell identifier (ID).

FIG. 6 shows an OFDM symbol for transmitting a synchronization signaland a PBCH within a radio frame in a frequency division duplex (FDD)system.

Referring to FIG. 6, a primary synchronization signal (PSS) istransmitted through last OFDM symbols of a slot #0 and a slot #10 withina frame. The same PSS is transmitted using 2 OFDM symbols. The PSS isused to obtain time domain synchronization such as OFDM symbolsynchronization, slot synchronization, or the like and/or frequencydomain synchronization. A Zadoff-Chu (ZC) sequence can be used as thePSS. At least one PSS exists in a wireless communication system.

A secondary synchronization signal (SSS) is transmitted through animmediately previous OFDM symbol from the last OFDM symbols of the slot#0 and the slot #10 within the frame. That is, the SSS and the PSS canbe transmitted through contiguous OFDM symbols. In addition, differentSSSs are transmitted through two OFDM symbols being transmitted. The SSSis used to obtain frame synchronization and/or cyclic prefix (CP)configuration of a cell, i.e., usage information of a normal CP or anextended CP. An m-sequence may be used as the SSS. One OFDM symbolincludes two m-sequences. For example, if one OFDM symbol includes 63subcarriers, two m-sequences each having a length of 31 are mapped toone OFDM symbol.

If a physical cell ID is denoted by N^(cell) _(ID), then N^(cell) _(ID)can be obtained by Equation 1 belowN ^(cell) _(ID)=3N ⁽¹⁾ _(ID) +N ⁽²⁾ _(ID)  [Equation 1]

Herein, N⁽²⁾ _(ID) denotes a physical layer ID as one of values rangesfrom 0 to 2, and is obtained by using the PSS. N⁽¹⁾ _(ID) denotes a cellgroup ID as one of values ranges from 0 to 167, and is obtained by usingthe SSS.

A physical broadcast channel (PBCH) is located at a subframe 0 (i.e., a1^(st) subframe) of a radio frame in a time domain. For example, thePBCH can be transmitted in a 2^(nd) slot of the subframe 0, i.e., firstfour OFDM symbols (i.e., from an OFDM symbol 0 to an OFDM symbol 3) of aslot 1. The PBCH can be transmitted by using the 72 consecutivesubcarriers in a frequency domain. The PBCH carries a limited number ofparameters which are most frequently transmitted and are essential forinitial cell access. A master information block (MIB) includes theseessential parameters. In the PBCH, each MIB transmission is spread witha period of 40 ms. That is, transmission is performed in fourconsecutive frames. This is to avoid missing of one entire MIB.

FIG. 7 shows a structure of an uplink subframe.

Referring to FIG. 7, the uplink subframe can be divided into a controlregion and a data region. A physical uplink control channel (PUCCH) forcarrying uplink control information (UCI) is allocated to the controlregion. A physical uplink shared channel (PUSCH) for carrying UL dataand/or the UCI is allocated to the data region. In this sense, thecontrol region can be called a PUCCH region, and the data region can becalled a PUSCH region. According to configuration information indicatedby a higher layer, a UE may support simultaneous transmission of thePUSCH and the PUCCH or may not support simultaneous transmission of thePUSCH and the PUCCH.

The PUSCH is mapped to an uplink shared channel (UL-SCH) which is atransport channel. UL data transmitted on the PUSCH may be a transportblock which is a data block for the UL-SCH transmitted during TTI. Thetransport block may be user information. Alternatively, the uplink datamay be multiplexed data. The multiplexed data may be attained bymultiplexing control information and the transport block for the UL-SCH.Examples of the UCI to be multiplexed to the uplink data include achannel quality indicator (CQI), a precoding matrix indicator (PMI), ahybrid automatic repeat request (HARQ)acknowledgement/not-acknowledgement (ACK/NACK), a rank indicator (RI), aprecoding type indication (PTI), etc. As such, when the UCI istransmitted in the data region together with the uplink data, it iscalled piggyback transmission of the UCI. Only the UCI may betransmitted through the PUSCH.

The PUCCH for one UE is allocated in an RB pair in a subframe. RBsbelonging to the RB pair occupy different subcarriers in each of a1^(st) slot and a 2^(nd) slot. A frequency occupied by the RBs belongingto the RB pair allocated to the PUCCH changes at a slot boundary. Thisis called that the RB pair allocated to the PUCCH is frequency-hopped atthe slot boundary. Since the UE transmits UCI on a time basis throughdifferent subcarriers, a frequency diversity gain can be obtained.

The UE generates a PUSCH signal through a process of scrambling,modulation, mapping to a transport layer, precoding, mapping to aresource element, generating of an SC-FDMA signal. In this case, asequence used in the scrambling is generated based on a UE-specific ID(i.e., an RNTI for the UE), and a physical cell ID.

Hereinafter, an uplink reference signal (RS) will be described.

In general, an RS is transmitted as a sequence. Any sequence can be usedas a sequence used for an RS sequence without particular restrictions.The RS sequence may be a phase shift keying (PSK)-based computergenerated sequence. Examples of the PSK include binary phase shiftkeying (BPSK), quadrature phase shift keying (QPSK), etc. Alternatively,the RS sequence may be a constant amplitude zero auto-correlation(CAZAC) sequence. Examples of the CAZAC sequence include a Zadoff-Chu(ZC)-based sequence, a ZC sequence with cyclic extension, a ZC sequencewith truncation, etc. Alternatively, the RS sequence may be apseudo-random (PN) sequence. Example of the PN sequence include anm-sequence, a computer generated sequence, a gold sequence, a Kasamisequence, etc. In addition, the RS sequence may be a cyclically-shiftedsequence.

The uplink RS can be classified into a demodulation reference signal(DM-RS) and a sounding reference signal (SRS). The DM-RS is an RS usedfor channel estimation to demodulate a received signal. The DM-RS can becombined with PUSCH or PUCCH transmission. The SRS is an RS transmittedfor uplink scheduling by a UE to a BS. The BS estimates an uplinkchannel by using the received SRS, and the estimated uplink channel isused in uplink scheduling. The SRS is not combined with PUSCH or PUCCHtransmission. The same type of base sequences can be used for the DM-RSand the SRS. Meanwhile, precoding applied to the DM-RS in uplinkmulti-antenna transmission may be the same as precoding applied to thePUSCH. Cyclic shift separation is a primary scheme for multiplexing theDM-RS. In a 3GPP LTE-A system, the SRS may not be precoded, and may bean antenna-specific RS.

An RS sequence r_(u,v) ^((α))(n) can be defined based on a base sequenceb_(u,v)(n) and a cyclic shift α according to Equation 2.r _(u,v) ^((α))(n)=e ^(jαn) b _(u,v)(n),0≦n<M _(sc) ^(RS)  [Equation 2]

In Equation 2, M_(sc) ^(RS) (1≦m≦N_(RB) ^(max,UL)) denotes an RSsequence length, where M_(sc) ^(RS)=m*N_(sc) ^(RB). N_(sc) ^(RB) denotesa size of a resource block represented by the number of subcarriers in afrequency domain. N_(RB) ^(max,UL) denotes a maximum value of an uplinkbandwidth expressed by a multiple of N_(sc) ^(RB). A plurality of RSsequences can be defined by differently applying a cyclic shift value αfrom one base sequence.

The base sequence is divided into a plurality of groups. In this case,uε{0, 1, . . . , 29} denotes a group index, and v denotes a basesequence index in a group. The base sequence depends on a base sequencelength M_(sc) ^(RS). Each group includes one base sequence (i.e., v=0)having a length of M_(sc) ^(RS) with respect to m (where 1≦m≦5), andincludes two base sequences (i.e., v=0, 1) having a length of M_(sc)^(RS) with respect to m (where 6≦m≦n_(RB) ^(max,UL)). The sequence groupindex u and the base sequence index v may vary over time similarly togroup hopping or sequence hopping to be described below.

In addition, if the RS sequence has a length greater than or equal to3N_(sc) ^(RB), the base sequence can be defined by Equation 3.b _(u,v)(n)=x _(q)(n mod N _(ZC) ^(RC)),0≦n<M _(sc) ^(RS)  [Equation 3]

In Equation 3, q denotes a root index of a Zadoff-Chu (ZC) sequence.N_(ZC) ^(RS) denotes a length of the ZC sequence, and may be given to amaximum prime number less than M_(sc) ^(RS). The ZC sequence with theroot index q can be defined by Equation 4.

$\begin{matrix}{{{x_{q}(m)} = e^{{- j}\frac{\pi\;{{qm}{({m + 1})}}}{N_{ZC}^{RS}}}},{0 \leq m \leq {N_{ZC}^{RS} - 1}}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack\end{matrix}$

q can be given by Equation 5.q=└q+½┘+v·(−1)^(└2q┘)q=N _(ZC) ^(RS)·(u+1)/31  [Equation 5]

If the length of the RS sequence is less than or equal to 3N_(sc) ^(RB),the base sequence can be defined by Equation 6.b _(u,v)(n)=e ^(jφ(n)π/4),0≦n≦M _(sc) ^(RS)−1  [Equation 6]

Table 1 shows an example of defining φ(n) when MscRS=NscRB.

TABLE 1 u φ (0), . . . , φ (11) 0 −1 1 3 −3 3 3 1 1 3 1 −3 3 1 1 1 3 3 3−1 1 −3 −3 1 −3 3 2 1 1 −3 −3 −3 −1 −3 −3 1 −3 1 −1 3 −1 1 1 1 1 −1 −3−3 1 −3 3 −1 4 −1 3 1 −1 1 −1 −3 −1 1 −1 1 3 5 1 −3 3 −1 −1 1 1 −1 −1 3−3 1 6 −1 3 −3 −3 −3 3 1 −1 3 3 −3 1 7 −3 −1 −1 −1 1 −3 3 −1 1 −3 3 1 81 −3 3 1 −1 −1 −1 1 1 3 −1 1 9 1 −3 −1 3 3 −1 −3 1 1 1 1 1 10 −1 3 −1 11 −3 −3 −1 −3 −3 3 −1 11 3 1 −1 −1 3 3 −3 1 3 1 3 3 12 1 −3 1 1 −3 1 1 1−3 −3 −3 1 13 3 3 −3 3 −3 1 1 3 −1 −3 3 3 14 −3 1 −1 −3 −1 3 1 3 3 3 −11 15 3 −1 1 −3 −1 −1 1 1 3 1 −1 −3 16 1 3 1 −1 1 3 3 3 −1 −1 3 −1 17 −31 1 3 −3 3 −3 −3 3 1 3 −1 18 −3 3 1 1 −3 1 −3 −3 −1 −1 1 −3 19 −1 3 1 31 −1 −1 3 −3 −1 −3 −1 20 −1 −3 1 1 1 1 3 1 −1 1 −3 −1 21 −1 3 −1 1 −3 −3−3 −3 −3 1 −1 −3 22 1 1 −3 −3 −3 −3 −1 3 −3 1 −3 3 23 1 1 −1 −3 −1 −3 1−1 1 3 −1 1 24 1 1 3 1 3 3 −1 1 −1 −3 −3 1 25 1 −3 3 3 1 3 3 1 −3 −1 −13 26 1 3 −3 −3 3 −3 1 −1 −1 3 −1 −3 27 −3 −1 −3 −1 −3 3 1 −1 1 3 −3 −328 −1 3 −3 3 −1 3 3 −3 3 3 −1 −1 29 3 −3 −3 −1 −1 −3 −1 3 −3 3 1 −1

Table 2 shows an example of defining φ(n) when M_(sc) ^(RS)=2*N_(sc)^(RB).

TABLE 2 u φ (0), . . . , φ (23) 0 −1 3 1 −3 3 −1 1 3 −3 3 1 3 −3 3 1 1−1 1 3 −3 3 −3 −1 −3 1 −3 3 −3 −3 −3 1 −3 −3 3 −1 1 1 1 3 1 −1 3 −3 −3 13 1 1 −3 2 3 −1 3 3 1 1 −3 3 3 3 3 1 −1 3 −1 1 1 −1 −3 −1 −1 1 3 3 3 −1−3 1 1 3 −3 1 1 −3 −1 −1 1 3 1 3 1 −1 3 1 1 −3 −1 −3 −1 4 −1 −1 −1 −3 −3−1 1 1 3 3 −1 3 −1 1 −1 −3 1 −1 −3 −3 1 −3 −1 −1 5 −3 1 1 3 −1 1 3 1 −31 −3 1 1 −1 −1 3 −1 −3 3 −3 −3 −3 1 1 6 1 1 −1 −1 3 −3 −3 3 −3 1 −1 −1 1−1 1 1 −1 −3 −1 1 −1 3 −1 −3 7 −3 3 3 −1 −1 −3 −1 3 1 3 1 3 1 1 −1 3 1−1 1 3 −3 −1 −1 1 8 −3 1 3 −3 1 −1 −3 3 −3 3 −1 −1 −1 −1 1 −3 −3 −3 1 −3−3 −3 1 −3 9 1 1 −3 3 3 −1 −3 −1 3 −3 3 3 3 −1 1 1 −3 1 −1 1 1 −3 1 1 10−1 1 −3 −3 3 −1 3 −1 −1 −3 −3 −3 −1 −3 −3 1 −1 1 3 3 −1 1 −1 3 11 1 3 3−3 −3 1 3 1 −1 −3 −3 −3 3 3 −3 3 3 −1 −3 3 −1 1 −3 1 12 1 3 3 1 1 1 −1−1 1 −3 3 −1 1 1 −3 3 3 −1 −3 3 −3 −1 −3 −1 13 3 −1 −1 −1 −1 −3 −1 3 3 1−1 1 3 3 3 −1 1 1 −3 1 3 −1 −3 3 14 −3 −3 3 1 3 1 −3 3 1 3 1 1 3 3 −1 −1−3 1 −3 −1 3 1 1 3 15 −1 −1 1 −3 1 3 −3 1 −1 −3 −1 3 1 3 1 −1 −3 −3 −1−1 −3 −3 −3 −1 16 −1 −3 3 −1 −1 −1 −1 1 1 −3 3 1 3 3 1 −1 1 −3 1 −3 1 1−3 −1 17 1 3 −1 3 3 −1 −3 1 −1 −3 3 3 3 −1 1 1 3 −1 −3 −1 3 −1 −1 −1 181 1 1 1 1 −1 3 −1 −3 1 1 3 −3 1 −3 −1 1 1 −3 −3 3 1 1 −3 19 1 3 3 1 −1−3 3 −1 3 3 3 −3 1 −1 1 −1 −3 −1 1 3 −1 3 −3 −3 20 −1 −3 3 −3 −3 −3 −1−1 −3 −1 −3 3 1 3 −3 −1 3 −1 1 −1 3 −3 1 −1 21 −3 −3 1 1 −1 1 −1 1 −1 31 −3 −1 1 −1 1 −1 −1 3 3 −3 −1 1 −3 22 −3 −1 −3 3 1 −1 −3 −1 −3 −3 3 −33 −3 −1 1 3 1 −3 1 3 3 −1 −3 23 −1 −1 −1 −1 3 3 3 1 3 3 −3 1 3 −1 3 −1 33 −3 3 1 −1 3 3 24 1 −1 3 3 −1 −3 3 −3 −1 −1 3 −1 3 −1 −1 1 1 1 1 −1 −1−3 −1 3 25 1 −1 1 −1 3 −1 3 1 1 −1 −1 −3 1 1 −3 1 3 −3 1 1 −3 −3 −1 −126 −3 −1 1 3 1 1 −3 −1 −1 −3 3 −3 3 1 −3 3 −3 1 −1 1 −3 1 1 1 27 −1 −3 33 1 1 3 −1 −3 −1 −1 −1 3 1 −3 −3 −1 3 −3 −1 −3 −1 −3 −1 28 −1 −3 −1 −1 1−3 −1 −1 1 −1 −3 1 1 −3 1 −3 −3 3 1 1 −1 3 −1 −1 29 1 1 −1 −1 −3 −1 3 −13 −1 1 3 1 −1 3 1 3 −3 −3 1 −1 −1 1 3

Hopping of the RS can be applied as follows.

A sequence group index u for each slot index n_(s) can be defined basedon a group hopping pattern f_(gh)(n_(s)) and a sequence shift patternf_(ss) according to Equation 7.u=(f _(gh)(n _(s))+f _(ss))mod 30  [Equation 7]

There may be 17 different group hopping patterns and 30 differentsequence shift patterns. Whether to apply group hopping may be indicatedby a higher layer.

The PUCCH and the PUSCH may have the same group hopping pattern. Thegroup hopping pattern f_(gh)(n_(s)) can be defined by Equation 8.

$\begin{matrix}{{f_{gh}( n_{s} )} = \{ \begin{matrix}0 & {{if}\mspace{14mu}{group}\mspace{14mu}{hopping}\mspace{14mu}{is}\mspace{14mu}{disabled}} \\{( {\sum\limits_{i = 0}^{7}{{c( {{8n_{s}} + i} )} \cdot 2^{i}}} ){mod}\mspace{14mu} 30} & {{if}\mspace{14mu}{group}\mspace{14mu}{hopping}\mspace{14mu}{is}\mspace{14mu}{enabled}}\end{matrix} } & \lbrack {{Equation}\mspace{14mu} 8} \rbrack\end{matrix}$

In Equation 8, c(i) is a PN sequence, i.e., a pseudo-random sequence.The PN sequence can be defined by a length-31 gold sequence. Equation 9shows an example of the gold sequence c(n).c(n)=(x ₁(n+N _(c))+x ₂(n+N _(c)))mod 2x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₁(n+1)+x ₁(n))mod 2  [Equation 9]

Herein, Nc is 1600, x(i) is a first m-sequence, and y(i) is a secondm-sequence. For example, the first m-sequence or the second m-sequencemay be initialized in each OFDM symbol according to a cell ID, a slotnumber in a radio frame, an OFDM symbol index in a slot, a CP type, etc.A pseudo-random sequence generator can be initialized as

$c_{init} = \lfloor \frac{N_{ID}^{cell}}{30} \rfloor$at the start of each radio frame.

The PUCCH and the PUSCH may have the same sequence shift pattern. Thesequence shift pattern of the PUCCH can be given as f_(ss)^(PUCCH)=N_(ID) ^(cell) mod 30. The sequence shift pattern of the PUSCHcan be given as f_(ss) ^(PUSCH)=(f_(ss) ^(PUCCH)+Δ_(ss)) mod 30, andΔ_(ss)ε{0, 1, . . . , 29} can be configured by a higher layer.

Sequence hopping can be applied only to an RS sequence having a lengthgreater than 6N_(sc) ^(RB). In case of an RS having a length less than6N_(sc) ^(RB), a base sequence index v in a base sequence group is givento 0. In case of an RS having a length greater than or equal to 6N_(sc)^(RB), a base sequence index v in a base sequence group of a slot indexn_(s) can be defined by Equation 10.

$\begin{matrix}{v = \{ \begin{matrix}{c( n_{s} )} & \begin{matrix}{{if}\mspace{14mu}{group}\mspace{14mu}{hopping}\mspace{14mu}{is}\mspace{14mu}{disabled}\mspace{14mu}{and}} \\{{sequence}\mspace{14mu}{hopping}\mspace{14mu}{is}\mspace{14mu}{enabled}}\end{matrix} \\0 & {otherwise}\end{matrix} } & \lbrack {{Equation}\mspace{14mu} 10} \rbrack\end{matrix}$

c(i) can be expressed by the example of Equation 9. Whether to apply thesequence hopping can be indicated by a higher layer. A pseudo-randomsequence generator can be initialized as

$c_{init} = {{\lfloor \frac{N_{ID}^{cell}}{30} \rfloor \cdot 2^{5}} + f_{ss}^{PUSCH}}$at the start of each radio frame.

A DM-RS sequence for the PUSCH can be defined by Equation 11.r _(PUSCH) ^((λ))(m·M _(sc) ^(RS) +n)=w ^((λ))(m)r _(u,v) ^((α) ^(λ)⁾(n)  [Equation 11]

In Equation 11, λ denotes a layer, and is any one of {0, 1, . . . ,v−1}. In addition, m=0, 1, and n=0, . . . , M_(sc) ^(RS)−1. M_(sc)^(RS)=M_(sc) ^(PUSCH), and a sequence r_(u,v) ^((α) ^(λ) ⁾(0), . . . ,r_(u,v) ^((α) ^(λ) ⁾(M_(sc) ^(RS)−1) is defined by Equation 2.

An orthogonal sequence w^((λ))(m) is given by [w^((λ))(0) w^((λ))(1)]=[11] for a DCI format 0 if a higher layer parameter (i.e.,Activate-DMRS-with OCC) is not configured or if a temporary C-RNTI isused to transmit the latest uplink-related DCI for a transport blockassociated with corresponding PUSCH transmission, and otherwise it isgiven by a cyclic shift field included in the latest uplink-related DCIfor the transport block associated with the corresponding PUSCHtransmission as shown in the following table.

TABLE 3 Cyclic Shift Field in uplink-related DCI n_(DMRS, λ) ⁽²⁾[w^((λ))(0) w^((λ))(1)] format λ = 0 λ = 1 λ = 2 λ = 3 λ = 0 λ = 1 λ = 2λ = 3 000 0 6 3 9 [1 1] [1 1] [1 −1] [1 −1] 001 6 0 9 3 [1 −1] [1 −1] [11] [1 1] 010 3 9 6 0 [1 −1] [1 −1] [1 1] [1 1] 011 4 10 7 1 [1 1] [1 1][1 1] [1 1] 100 2 8 5 11 [1 1] [1 1] [1 1] [1 1] 101 8 2 11 5 [1 −1] [1−1] [1 −1] [1 −1] 110 10 4 1 7 [1 −1] [1 −1] [1 −1] [1 −1] 111 9 3 0 6[1 1] [1 1] [1 −1] [1 −1]

In the slot n_(s), a cyclic shift value is given as α_(λ)=2πn_(cs,λ)/12,and n_(cs,λ), can be defined by Equation 12.n _(cs,λ)=(n _(DMRS) ⁽¹⁾ +n _(DMRS,λ) ⁽²⁾ +n _(PN)(n _(s)))mod12  [Equation 12]

In Equation 12, n⁽²⁾ _(DMRS,λ) denotes a value given in Table 3 aboveaccording to a cyclic shift field for a DMRS included in the latestuplink-related DCI for the transport block associated with thecorresponding PUSCH transmission, and n⁽¹⁾ _(DMRS) denotes a value givenin Table 4 below according to a parameter ‘cyclicShift’ provided by ahigher layer signal.

TABLE 4 cyclicShift n_(DMRS) ⁽¹⁾ 0 0 1 2 2 3 3 4 4 6 5 8 6 9 7 10

n_(PN)(n_(s)) is given by the following equation.n _(PN)(n _(s))=Σ_(i=0) ⁷ c(8N _(symb) ^(UL) ·n _(s)+i)·2^(i)  [Equation 13]

In Equation 13, a pseudo-random sequence c(i) is defined by Equation 9.A pseudo-random sequence generator can be initialized as

$c_{init} = {{\lfloor \frac{N_{ID}^{cell}}{30} \rfloor \cdot 2^{5}} + f_{ss}^{PUSCH}}$at the start of each radio frame.

A vector of RSs can be precoded by the following equation.

$\begin{matrix}{\begin{bmatrix}{\overset{\sim}{r}}_{PUSCH}^{(0)} \\\vdots \\{\overset{\sim}{r}}_{PUSCH}^{({P - 1})}\end{bmatrix} = {W\begin{bmatrix}r_{PUSCH}^{(0)} \\\vdots \\r_{PUSCH}^{({\upsilon - 1})}\end{bmatrix}}} & \lbrack {{Equation}\mspace{14mu} 14} \rbrack\end{matrix}$

In Equation 14, P denotes the number of antenna ports used for PUSCHtransmission. For PUSCH transmission using a single-antenna port, P=1,W=1, and v=1.

For spatial multiplexing, P=2 or P=4. A precoding matrix W may beidentical to a precoding matrix used for the PUSCH in the same subframe.

As described above with reference to Equations 2 to 14, the existingDM-RS is generated based on a physical cell ID in a process of basesequence generation, and group hopping and sequence hopping.

The DM-RS generated through the aforementioned process is transmittedafter being mapped to a physical resource.

FIG. 8 shows an exemplary structure of a subframe in which an RS istransmitted.

The subframe structure of FIG. 8-(a) is for a normal CP case. Thesubframe includes a 1^(st) slot and a 2^(nd) slot. Each of the 1^(st)slot and the 2^(nd) slot includes 7 SC-FDMA symbols. 14 SC-FDMA symbolsin the subframe are indexed from 0 to 13. The RS can be transmitted byusing SC-FDMA symbols indexed from 3 to 10. The RS can be transmitted byusing a sequence. A ZC sequence can be used as an RS sequence. VariousZC sequences can be generated according to a root index and a cyclicshift value. A BS can estimate a channel of a plurality of UEs throughan orthogonal sequence or a quasi-orthogonal sequence by allocating adifferent cyclic shift value to the UE. A location of a frequency domainoccupied by the RS may be identical or different in two slots in thesubframe. The same RS sequence is used in the two slots. Data can betransmitted through the remaining SC-FDMA symbols other than an SC-FDMAsymbol in which the RS is transmitted. The subframe structure of FIG.8-(b) is for an extended CP case. The subframe includes a 1^(st) slotand a 2^(nd) slot. Each of the 1^(st) slot and the 2^(nd) slot includes6 SC-FDMA symbols. 12 SC-FDMA symbols in the subframe are indexed from 0to 11. The RS is transmitted through SC-FDMA symbols indexed from 2 to8. Data is transmitted through the remaining SC-FDMA symbols other thanan SC-FDMA symbol in which the RS is transmitted.

In MU-MIMO transmission, the same frequency band is allocated tomultiple UEs by using a PUSCH resource when using the conventionalmethod. In addition, when generating a DM-RS sequence, each UE applies adifferent cyclic shift value α and orthogonal code cover (OCC) value.According to this method, the most orthogonal DM-RS sequences aretransmitted between the UEs. However, there are many UEs in a multi-nodesystem, and each UE may have different uplink channel quality and adifferent uplink signal transmission amount. Therefore, it may berequired to allocate a PUSCH resource having a different number ofresource blocks to each UE.

For this, PUSCH resources each having a different number of resourceblocks can be allocated to respective UEs, and there may be anoverlapping (duplicated) region between the allocated PUSCH resources.That is, scheduling can be achieved such that MU-MIMO transmission isperformed only in some of the PUSCH regions allocated to the respectiveUEs. In this case, if the UEs to which the overlapping PUSCH resourcesare allocated generate a DM-RS according to the conventional method,orthogonality is significantly impaired between sequences constitutingthe DM-RS.

FIG. 9 shows a DM-RS transmission method according to an embodiment ofthe present invention. It is assumed that UEs #1 and #2 are UEs whichoperate based on MU-MIMO.

Referring to FIG. 9, a BS or a node transmits a physical ID to the UEs#1 and #2 by using a synchronization signal (step S101).

The BS or the node transmits a parameter for a virtual cell ID #1 to theUE #1 by using a higher layer signal (step S102). In addition, the BS orthe node transmits a parameter for a virtual cell ID #2 by using ahigher layer signal (step S103). Herein, the virtual cell ID is a cellID which is virtual and which is provided for each UE, and differs fromthe physical cell ID. The virtual cell ID may be used when the UEgenerates a DM-RS.

The parameter for the virtual cell ID may be plural in number, and forexample, may be N⁽¹⁾ _(ID) and N⁽²⁾ _(ID) used to generate the physicalcell ID.

Although an example of transmitting the parameter for the virtual cellID by using the higher layer signal such as a radio resource control(RRC) message is described in FIG. 9, the present invention is notlimited thereto. That is, the parameter for the virtual cell ID may betransmitted by being included in physical layer control information,i.e., DCI.

The BS or the node transmits first uplink scheduling information to theUE #1 (step S104). The BS or the node transmits second uplink schedulinginformation to the UE #2 (step S105). Herein, by using the first uplinkscheduling information and the second uplink scheduling information,PUSCHs each having a different number of resource blocks may bescheduled for the UEs #1 and #2 in such a manner that some of theresource blocks overlap with each other.

The UE #1 transmits an uplink signal by using the physical cell ID, andtransmits the DM-RS by using the virtual cell ID #1 (step S106). Forexample, the uplink signal which uses the physical cell ID may be anSRS. The UE #2 transmits the uplink signal by using the physical cellID, and transmits the DM-RS by using the virtual cell ID #2 (step S107).

That is, each UE may generate and transmit some uplink signals by usingthe same physical cell ID, and may generate and transmit the DM-RS byusing different virtual cell IDs. That is, to generate the DM-RS, the UEuses the virtual cell ID instead of a physical cell ID N^(cell) _(ID) insome or all of the aforementioned Equations 2 to 13. In other words, UEsexisting in the same cell generate a DM-RS sequence by using differentvirtual cell IDs. In this case, the number of resource blocks allocatedto the PUSCHs may differ between the UEs #1 and #2, and the DM-RS may betransmitted only in an allocated PUSCH region. Therefore, orthogonalityis not completely maintained between the DM-RS sequences. However, thismethod provides a better performance than a case where a DM-RS sequencegenerated by using the same physical cell ID is distinguished by using acyclic shift or an OCC and is then transmitted by UEs to which thedifferent numbers of resource blocks are allocated. A process ofgenerating a DM-RS by each UE by using a parameter for a virtual cell IDwill be described in greater detail.

The virtual cell ID can replace a physical cell ID in a part or entiretyof the sequence generation, group hopping and sequence hopping processof the DM-RS described in Equations 2 to 13. As described above, asequence used as the DM-RS is generated by cyclically shifting a basesequence selected from one sequence group among a plurality of sequencegroups. Each of the plurality of sequence groups includes one basesequence.

In addition, the DM-RS is transmitted in at least two slots in a frameincluding a plurality of slots in a time domain. In this case, onesequence group is selected for each slot of the slots in which the DM-RSis transmitted. This process is called group hopping. In addition, onebase sequence is selected from the selected one sequence group, and thisprocess is called sequence hopping.

If each of the aforementioned three processes has a different value forreplacing the existing physical cell ID, a plurality of parameters canbe configured for the parameter for the virtual cell ID.

For example, if a cell ID to be used when the base sequence of the DM-RSis generated differs from a cell ID to be used in DM-RS sequencehopping, the parameter for the proposed virtual cell ID may include aplurality of cell IDs or a plurality of parameters for replacing thecell IDs.

The parameter for the proposed virtual cell ID may include anotherparameter that can replace a value in association with a physical cellID in the conventional DM-RS generation process in addition to a virtualcell ID having an integer value ranges from 0 to 503 in the same manneras the physical cell ID. In the conventional DM-RS generation process,the physical cell ID has an effect when generating a value c_(init) usedin the sequence hopping process, a value c_(init) and sequence-shiftpattern value f_(ss) used in the sequence hopping process, a valuec_(init) used in the DM-RS sequence generation process, etc. That is,the three types of the values c_(init), the value f_(ss), etc., aredetermined by the physical cell ID in the conventional method, whereasare determined by the parameter for the virtual cell ID according to themethod proposed in the present invention. Therefore, the parameter forthe virtual cell ID may include not only the virtual cell ID but alsosome of the three types of values c_(init) and the value f_(ss). Thethree types of values c_(init) each have different generation equations,and thus may be independently included in the parameter for the virtualcell ID. The parameter for the virtual cell ID proposed according to oneembodiment may include a virtual cell ID for replacing the physical cellID in the DM-RS sequence hopping and group hopping process and a valuec_(init) for replacing a value c_(init) determined by the physical cellID in the DM-RS sequence generation process.

The proposed virtual cell ID may be used when generating at least one ofa PUSCH DM-RS and a PUCCH DM-RS. A different virtual cell ID may be usedto generate each DM-RS.

The BS may report in advance information indicating whether the UEgenerates the DM-RS by using the physical cell ID or generates the DM-RSby using a parameter for the virtual cell ID, by adding the informationto DCI or a higher layer signal such as an RRC message. Although only anexample in which each UE performing MU-MIMO transmits the DM-RS by usingthe virtual cell ID is described in FIG. 8, the present invention is notlimited thereto. That is, each UE may additionally transmit anotheruplink signal in addition to the DM-RS among uplink signals, by usingthe virtual cell ID.

FIG. 10 shows a structure of a BS and a UE according to an embodiment ofthe present invention.

A BS 100 is an example of a node. The BS 100 includes a processor 110, amemory 120, and a radio frequency (RF) unit 130. The processor 110implements the proposed functions, procedures, and/or methods. Forexample, the processor 110 transmits a parameter for a virtual cell IDto a UE by using a higher layer signal or a physical layer signal, andtransmits scheduling information. The scheduling information may bescheduled such that MU-MIMO is performed in some regions of a PUSCHradio resource to which a plurality of UEs are allocated. In addition,the processor 110 reports a physical cell ID by using a synchronizationsignal. The memory 120 is coupled to the processor 110, and stores avariety of information for driving the processor 110. The RF unit 130 iscoupled to the processor 110, and transmits and/or receives a radiosignal.

A UE 200 includes a processor 210, a memory 220, and an RF unit 230. Theprocessor 210 implements the proposed functions, procedures, and/ormethods. For example, the processor 210 receives a physical cell ID fromthe BS by using a synchronization signal, and receives a parameter for avirtual cell ID by using a higher layer signal or a physical layersignal. The parameter for the virtual cell ID may be used to generatethe virtual cell ID, and the virtual cell ID may be used to generate anuplink DM-RS sequence. That is, the processor 210 may generate someuplink signals by using the physical cell ID, and may generate theremaining uplink signals by using the virtual cell ID. The physical cellID may be cell-specific (i.e., it may be specific for each cell), andthe virtual cell ID may be node-specific (i.e., another node in the samecell may have a different virtual cell ID). The memory 220 is coupled tothe processor 210, and stores a variety of information for driving theprocessor 210. The RF unit 230 is coupled to the processor 210, andtransmits and/or receives a radio signal.

The processors 110 and 210 may include an application-specificintegrated circuit (ASIC), a separate chipset, a logic circuit, a dataprocessing unit, and/or a converter for mutually converting a basebandsignal and a radio signal. The memories 120 and 220 may include aread-only memory (ROM), a random access memory (RAM), a flash memory, amemory card, a storage medium, and/or other equivalent storage devices.The RF units 130 and 230 may include one or more antennas fortransmitting and/or receiving a radio signal. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememories 120 and 220 and may be performed by the processors 110 and 210.The memories 120 and 220 may be located inside or outside the processors110 and 210, and may be coupled to the processors 110 and 210 by usingvarious well-known means.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. Therefore, the scope of theinvention is defined not by the detailed description of the inventionbut by the appended claims, and all differences within the scope will beconstrued as being included in the present invention.

What is claimed is:
 1. A method for transmitting demodulation referencesignals, performed by a user equipment, the method comprising: receivingsynchronization signals including a primary synchronization signal and asecondary synchronization signal; transmitting a first demodulationreference signal based on a physical cell identity (ID) which isdetermined by the synchronization signals; receiving a higher layersignal; and transmitting a second demodulation reference signal based ona virtual cell ID which is configured by the higher layer signal,wherein the second demodulation reference signal is transmitted in twoslots in a subframe, one sequence group is selected in each of the twoslots and the second demodulation reference signal is generated bycyclically shifting one base sequence selected in the selected sequencegroup, and wherein the one base sequence is determined based on thevirtual cell ID.
 2. The method of claim 1, wherein the higher layersignal is a radio resource control (RRC) message.
 3. The method of claim1, wherein the second demodulation reference signal is transmitted at afourth and an eleventh single carrier-frequency division multiple access(SC-FDMA) symbols in an uplink subframe including 14 SC-FDMA symbols. 4.The method of claim 1, wherein the second demodulation reference signalis transmitted at third and ninth single carrier-frequency divisionmultiple access (SC-FDMA) symbols in an uplink subframe including 12SC-FDMA symbols.
 5. The method of claim 1, wherein the seconddemodulation reference signal is a reference signal which is associatedwith a physical uplink shared channel (PUSCH).
 6. A user equipment (UE)for transmitting demodulation reference signals, the UE comprising: aradio frequency (RF) unit configured to transmit and receive a radiosignal; and a processor coupled to the RF unit and configured to:receive synchronization signals including a primary synchronizationsignal and a secondary synchronization signal, transmit a firstdemodulation reference signal based on a physical cell identity (ID)which is determined by the synchronization signals, receive a higherlayer signal, and transmit a second demodulation reference signal basedon a virtual cell ID which is configured by the higher layer signal,wherein the second demodulation reference signal is transmitted in twoslots in a subframe, one sequence group is selected in each of the twoslots and the second demodulation reference signal is generated bycyclically shifting one base sequence selected in the selected sequencegroup, and wherein the one base sequence is determined based on thevirtual cell ID.
 7. The UE of claim 6, wherein the higher layer signalis a radio resource control (RRC) message.
 8. The UE of claim 6, whereinthe second demodulation reference signal is transmitted at a fourth andan eleventh single carrier-frequency division multiple access (SC-FDMA)symbols in an uplink subframe including 14 SC-FDMA symbols.
 9. The UE ofclaim 6, wherein the second demodulation reference signal is transmittedat third and ninth single carrier-frequency division multiple access(SC-FDMA) symbols in an uplink subframe including 12 SC-FDMA symbols.10. The UE of claim 6, wherein the second demodulation reference signalis a reference signal which is associated with a physical uplink sharedchannel (PUSCH).